Ultrasonic testing is a type of non-destructive testing (NDT) whereby ultrasonic waves are propagated within the material or object to be tested. Defects or incongruities within the material may change the way that the ultrasonic waves are transmitted through the material or reflected off the material. Those changes in transmission/reflection are detected during ultrasonic testing, providing a diagnostic for defects within the material. Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites. It is used in many industries including steel and aluminum construction, metallurgy, manufacturing, aerospace, automotive and other transportation sectors.
Conventional ultrasonic imaging methods involve either an immersion or contact ultrasonic probe(s) to scan, mechanically or electronically, over an area of interest. There are two different sending and receiving modes; the pulse-echo and the pitch-catch. In the pulse-echo mode setup, a single ultrasonic probe is used to send and receive ultrasonic waves. In the case of pitch-catch mode, two ultrasonic probes are involved; one for sending and the other for receiving. In this case, both sending and receiving probes can be arranged to be on the same side of the test specimen or can be arranged to be on the opposite sides to each other. The later arrangement is often referred to as a “through-transmission mode” because the receiving probe detects only the transmitted part of ultrasonic waves through the test specimen.
Regardless of the setup mode used for a nondestructive testing, the received signals contain ultrasonic information about the beam scattering, diffraction and reflection that can occur inside of the test specimen in the form of amplitude variations and/or sound velocity changes when a burst of ultrasonic waves interacts with the internal features of the test specimen in an immersion setup. These variations in signals can be captured by a data acquisition unit and recorded on a computer along with the corresponding position data to generate a mapped image of the preprogramed scan area.
A mapped ultrasonic image over an area is called a C-scan ultrasonic image and is commonly used in a nondestructive testing process to visually represent and interpret the size and shape of the internal features after a scan is completed. This collective information via an imaging process makes it much easier to understand features of a test specimen rather than trying to interpret the electronic response signals of all data points (A-scan data). The visual quality of a C-scan image is determined by the spatial resolution used to take A-scan data, i.e. how small of scan step is used to move to the next data point. Each A-scan data represents a pixel in a C-scan image. This is a similar definition as the pixel size of a digital camera, where each scan step taken to collect an ultrasonic image corresponds to each pixel of the sensor in a digital camera. Again, the visual resolution of a final C-scan image depends on the step size of the scan. The smallest scan step that can be taken to collect ultrasonic A-scan data is limited by the accuracy and the resolution of the mechanical scanner used in an imaging system, which are typically a few micrometers for a high resolution translational stage.
In addition to the requirement of a fine scan step size to generate a high-resolution C-scan image, it is also desirable to have a highly concentrated ultrasonic beam over a small area to increase the probability of ultrasonic interaction with submillimeter scale internal features. In an immersion scanning method, it is common to use a focused ultrasonic probe to make the beam as small as possible at a given operating frequency. The beam size of a focused probe may be on the order of a few millimeters for the frequency range between 1 MHz and 10 MHz. This means that the ultrasonic interaction with small internal features strongly depends on the ratio between the beam diameter and the size of a feature to be detected. Once again, the minimum detectable feature size depends on the focal diameters of the sending and receiving ultrasonic probes. For example, if an internal feature is 0.1 mm in size and the ultrasonic beam diameter is 2 mm at the focal point, the feature is twenty times smaller and hence the detectability would be low due to a small variation in the ultrasonic signal.
In the case of the aforementioned example, the amplitude of reflected or scattered signal related to the tiny feature would be roughly in the order of ˜5% of the amplitude of the incident waves. This 5% change in amplitude would give a low signal-to-noise ratio for the feature signal, meaning that distinguishing the feature from the background noise would be difficult. In many cases, a signal averaging method (either summed or continuous) is used to reduce the background noise, which helps to increase the signal-to-noise ratio for a weak feature signal. Of course, this averaging process prolongs the overall scanning time significantly because each data point needs to be averaged.
Another aspect that should to be considered in ultrasonic scanning is the relationship between the scan step size and the ultrasonic beam diameter. When an aerial scan is performed with a relatively small step size of 0.02 mm (5 times smaller than the exemplary 0.1 mm size feature), no significant changes in ultrasonic signals would occur over the entire 2 mm beam diameter as the beam passes over the 0.1 mm diameter feature because of the large difference between the feature size and the beam diameter. Thus, the scanned image of the 0.1 mm feature would be oversized as well as faint even with a relative small scan step size of 0.02 mm is used. In this case, the small scan step has no meaningful benefit since the ultrasonic beam diameter is too big for the feature.
In addition to the scan step size and the ratio of focused beam diameter versus feature size, the sensitivity of ultrasonic beam to internal features such as pores, cracks, inclusions, lack of fusion or dis-bond can also depend on the wavelength of the ultrasonic waves propagating through the test specimen. In NDT, it is a common practice to select the right frequency ultrasonic probe based on both the minimum detectable feature size and the wavelength of the ultrasonic waves in the medium to be tested. For example, 10 MHz longitudinal mode ultrasonic waves in a typical carbon steel material have about 0.6 mm wavelength. Therefore, the minimum detectable size of internal features with a high confidence in steel would be about 0.3 mm (half of the wavelength). In other words, any internal features that are smaller than 0.3 mm would not appear clear in a C-scan image and would be difficult to interpret.
All three different aspects—the scan step size, beam diameter, and the wavelength—should be considered at the same time to generate a high-quality C-scan image. There are scanning acoustic microscopy systems that can generate microscopic resolution images based on the Rayleigh surface wave propagation theory. However, these systems are designed to examine the surface within a few micrometers deep, rather than for volumetric features. For volumetric internal features, it is necessary to use bulk waves (shear or compressional mode waves) that can penetrate through the material under testing. In addition, these microscopy systems utilize special ultrasonic probes designed to operate at a frequency of several hundreds of megahertz.
Commercially available conventional ultrasonic imaging systems that utilize either phased array or single-element ultrasonic probes in an immersion setup are limited because they do not form a microscopic sized focused beam capable of resolving microscopic internal features. Another method of generating high resolution ultrasonic images uses a laser vibrometer. Such systems can be used to detect ultrasonic waves at a microscopic level. Since the laser beam emitted from a laser vibrometer can be focused down to approximately 10 micrometers in diameter, the laser beam can be scanned over an area using a microscopic scan step. This approach is usually taken with ultrasonic energy that is either induced using a contact probe or a stick-on type piezoelectric plate. Again, the limitation in resolving small microscopic features within a test material derives from the ultrasonic signal source. The ultrasonic wave generated by a contact probe is not focused, rather, it spreads out widely over the entire test area, which makes it difficult to detect the direct ultrasonic response of the internal microscopic feature. Typically, a laser vibrometer ultrasonic imaging system generates high resolution ultrasonic images revealing beam scattering phenomena.
It is apparent that a need exists for a nondestructive testing method whereby ultrasonic bulk waves can be used to generate a microscopic resolution ultrasonic C-scan image.